1. Field of the Invention
This invention relates to a method for forming a polarization-inverted portion such as a periodical polarization-inverted structure usable for a second harmonic wave-generation (SHG) device utilizing a Quasi-Phase-Matching (QPM) system.
2. Related Art Statement
As a blue laser-light source usable for an optical pickup, etc., a SHG device utilizing a QPM system, having an optical waveguide constructed from a periodical polarization-inverted portion in a substrate made of a ferroelectric single crystal such as a lithium-niobate single crystal or a lithium tantalate single crystal, is expected. The device may be widely used for optical disk memory, medicine, optochemical field, and various optical measurement.
In order to realize a high conversion efficiency in the SHG device, it is required that the above polarization-inverted portion is formed deeply in the ferroelectric single crystal substrate. The deeply forming method of the polarization-inverted portion is, for example, disclosed in the Kokai publication Kokai Hei 9-218431 (JPA9-218431). In the publication, a conventional voltage applying method is improved, and attempt is made to grow a polarization-inverted portion deeply from a main surface of a ferroelectric single crystal substrate by sloping the main surface thereof from the polarization axis thereof by 3 degrees.
Moreover, in the Kokai publication Kokai Hei 11-72809 (JP A 11-72809), a main surface of a ferroelectric single crystal substrate is inclined from a polarization axis of the ferroelectric single crystal substrate by 3 degrees, and a ctenoid electrode and a virgate electrode are formed on the main surface, and several low electric resistance portions are formed in between the forefronts of the ctenoid electrode and the virgate electrode. Then, when a direct current is applied between the ctenoid electrode and the virgate electrode, polarization-inverted portions are formed corresponding to the electrode pieces of the ctenoid electrode and the low electric resistance portions.
As mentioned above, the forming method of the polarization-inverted portion described in the Kokai publication Kokai Hei 11-72809 can certainly form the polarization-inverted portions corresponding to the electrode pieces of the ctenoid electrode and the low electric resistance portions. However, since a given space is placed in between the forefronts of the electrode pieces of the ctenoid electrode and the low electric resistance portions and in between the adjacent low electric resistance portions, the above polarization-inverted portions have given spaces therebetween. That is, the polarization-inverted portions are formed in separation. If a ferroelectric single crystal substrate having the above-mentioned periodical polarization-inverted portion is employed for a SHG device utilizing a QPM system, a fundamental wave interacts with only the forward polarization-inverted portions, that is, the polarization-inverted portions corresponding to the electrode pieces of the ctenoid electrode. Therefore, the conversion efficiency for a second harmonic wave may not be enhanced.
It is an object of the present invention to provide a new method for forming a polarization-inverted portion deeply from a main surface of a ferroelectric single crystal substrate having a single polarized domain.
This invention relates to a method for forming a polarization-inverted portion comprising the steps of:
preparing a substrate made of a ferroelectric single crystal,
fabricating a first electrode and a second electrode on a main surface of the substrate in separation,
applying a first voltage between the first electrode and the second electrode to generate and grow a first polarization-inverted portion toward the second electrode from the first electrode,
changing the distance between the first electrode and the second electrode, and
applying a second voltage between the first electrode and the second electrode to generate and grow a second polarization-inverted portion, in a different area from that of the first polarization-inverted portion, toward the second electrode from the first electrode.
The inventors have conceived that after some polarization-inverted portions (first polarization-inverted portions) are formed by a voltage applying method using a first electrode and a second electrode, additional polarization-inverted portions (second polarization-inverted portions) are formed in the different area by changing the distance of between the first electrode and the second electrode. The first polarization-inverted portion and the second polarization-inverted portion have their respective different depth for a main surface of a ferroelectric single crystal substrate. Therefore, if the distance between the first electrode and the second electrode is adjusted appropriately, the overlapping degree of the first polarization-inverted portion and the second polarization-inverted portion can be controlled. A conventional forming method of a polarization-inverted portion as described in the Kokai publication Kokai Hei 11-72809 can not control the overlapping degree of the polarization-inverted portions.
Herein, the xe2x80x9cvoltage applying methodxe2x80x9d means a method that a first electrode and a second electrode are provided on a main surface of a ferroelectric single crystal substrate so as to be opposed each other, and a given voltage is applied between the first electrode and the second electrode on the condition that the first electrode is positive, thereby to grow a polarization-inverted portion along the polarization axis from the first electrode. The voltage applying method is explained in the Kokai publications Kokai Hei 9-218431 and Kokai Hei 11-72809.
In a preferred embodiment of the present invention, the first electrode is constructed from a ctenoid electrode having plural strip electrode pieces, and the distance between the first electrode and the second electrode is changed by adjusting the lengths of the electrode pieces after the first polarization-inverted portion is formed. In this case, a desired periodical polarization-inverted structure can be formed easily.
The distance between the first electrode and the second electrode can be increased or decreased. In the case of increasing the distance, it is desired that a part of at least one of the first electrode and the second electrode is removed by etching treatment or dicer processing after the first polarization-inverted portion is formed. Or another first electrode or another second electrode may be formed, by conventional photolithography technique, on the main surface of the ferroelectric single crystal substrate after the above first electrode or the above second electrode is removed entirely.
Particularly, in light of the number of the processing steps for the ferroelectric single crystal substrate, the former distance-increasing method is preferable because it requires not so many processing steps. In the case of using the ctenoid electrode as the first electrode, the distance between the first electrode and the second electrode can be easily increased by shortening the electrode pieces of the ctenoid electrode through etching treatment or dicer processing.
In the case of forming the second polarization-inverted portion on the condition that the distance between the first electrode and the second electrode is increased, the electric potential of the first electrode may be set to be almost equal to that of the first electrode in the case of forming the first polarization-inverted portion. However, in this case, the second polarization-inverted portion is likely to be formed small due to the increased distance between the first electrode and the second electrode. Therefore, in order to develop the size of the first polarization-inverted portion, the electric potential of the first electrode in forming the second polarization-inverted portion is set to be larger than that of the first electrode in forming the first polarization-inverted portion.
In the case of forming the second polarization-inverted portion on the condition that the distance between the first electrode and the second electrode is decreased, the electric potential of the first electrode can be also set to be almost equal to that of the first electrode in the case of forming the first polarization-inverted portion. However, in this case, the second polarization-inverted portion is likely to be large due to the decreased distance between the first electrode and the second electrode. Therefore, in order to reduce the size of the second polarization-inverted portion, the electric potential of the first electrode in forming the second polarization-inverted portion is set to be smaller than that of the first electrode in forming the first polarization-inverted portion.
As mentioned above, the size of the first polarization-inverted portion or the second polarization-inverted portion can be controlled by adjusting the voltage between the first electrode and the second electrode even though the distance between the first electrode and the second electrode is changed.
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